Roofing granules are coated with opaque pigments in order to shield asphaltic roofing materials from ultraviolet radiation and also to provide an esthetically pleasing appearance. Because pigments tend to be much more expensive than the starting granules, it is important to keep the pigment proportion to the minimum that will provide both the desired ultraviolet protection and esthetic appearance. It is even more important to maintain uniformity of color from batch to batch produced over prolonged periods of time. Accordingly, colorimeter tests are periodically performed while pigmenting the granules. When the granules are being coated with a mixture of pigments, the proportion of each pigment must be carefully controlled and verified by periodic testing to achieve the desired coloration.
In the prior art, colorimeter samples have been prepared by overfilling a dish with a mass of loose roofing granules and flattening the surface of the mass with a flat object. Only operators of considerable skill and experience are able to obtain a surface uniformity providing reasonably uniform colorimeter measurements.
The same problem of obtaining reproducible colorimeter measurements has been encountered in the production of other types of loose particles, for example, the pigments themselves, which either may be dry or dispersed in a liquid when being checked for color. Among other types of loose particles which need to be checked for color uniformity are colored glass beads.